1. Field of the Invention
The present invention relates to a superconducting current lead for supplying a current from an electric source placed in a room temperature to a superconducting apparatus such as a superconducting magnet cooled to a very low temperature.
2. Description of the Related Art
Superconducting magnets are used for physical property researches and for magnetic resonance systems, etc., and they are intended to be applied to magnetic levitated trains, nuclear-fusion magnetic containment systems, etc. in the future. In each of these superconducting magnets placed in a very low temperature, there arises a problem that heat enters a very low temperature region when the superconducting magnet is supplied with a current from an electric source placed in a room temperature. For example, liquid helium is an expensive liquid, which is not less than 1000 yen per liter. Refrigerator-input electric power necessary for a refrigerator to re-liquefy helium vaporized by heat entrance of 1 W is about 400 W even in an ideal state, but it reaches 1000 W in actual circumstances. For this reason, if there is a large quantity of heat entrance through a current lead, not only the cost of liquid helium, is increased but also the size or capacity of the refrigerator required for re-liquefying helium is increased. Hence, Such a situation makes the current leads meaningless since a superconducting magnet system including the current leads aims at the size reduction and power saving.
Therefore, the development of a low heat-entrance type current lead has become an important theme. After an oxide superconducting material was found out, a current lead using the oxide superconducting material in the low-temperature side of the current lead has been developed for reducing the quantity of heat entrance into a very low-temperature portion. For example, JP-A-64-76707 discloses such a superconducting current lead as follows. An intermediate portion of a current lead for supplying a current to a superconducting apparatus in liquid helium is used as a thermal anchor for cooling the portion of the current lead to the temperature of liquid nitrogen. A superconductor (such as Y--Ba--Cu--O) having a critical temperature not lower than the boiling point (78 K) of liquid nitrogen is used as a material for a lead portion lower than the thermal anchor portion. The temperature of the superconductor portion is always kept so as to be not higher than the critical temperature to thereby prevent the destruction of superconductivity. Further, JP-A-5-109530 discloses such a superconducting current lead as follows. The superconducting current lead is constituted by various kinds of conductors; for example, three kinds of conductors each disposed in a low-temperature portion, in an intermediate-temperature portion and in a high-temperature portion. Connection members in their longitudinal direction connect these portions. The low-temperature portion, the intermediate-temperature portion and the high-temperature portion are different from one another in shape, superconducting characteristic (critical current density (Jc), critical temperature, resistance value at the time of current conduction with current density larger than Jc, and their dependencies on the magnetic field or temperature) and structure. For example, conductor materials for the low-temperature portion, the intermediate-temperature portion and the high-temperature portion are as follows. A material in which a Bi-based oxide superconductor layer having high critical current density at 4.2 K is provided, through a thin layer of Ag, on an electrically insulating substrate having low heat-conductivity as a reinforcing material, is used as a conductor of the low-temperature portion. A conductor, in which tape-like wires each having a core of an oxide superconductor coated with a coating material are laminated and collected, is used as a conductor of the intermediate-temperature portion. An Y-based oxide superconductor having high critical current density (Jc) at 77 K is used as the core in the intermediate-temperature portion. A Au alloy containing a small amount of Pd is used as the coating material in the intermediate-temperature portion in order to suppress heat entrance. A conductor, in which tape-like wires are laminated and collected in the same manner as in the intermediate-temperature portion, is used as a conductor of the high-temperature portion. A Tl-based oxide superconductor having a high critical temperature is used as a core of the high-temperature portion. Ag having a small resistance value is used as a coating material of the high-temperature portion. In this manner, a material having low heat-conductivity is used in the vicinity of the superconducting magnet to thereby reduce heat entrance caused by heat conduction. Further, JP-A-4-218215 discloses such a superconducting current lead as follows. Silver-sheathed oxide superconductors and a pipe of FRP or a pipe of a metal such as silver, copper, aluminum, nickel, stainless steel, or the like, or an alloy of these metals as a support member to be combined with the oxide superconductor are provided. The oxide superconductors are bonded to the pipe by an adhesive agent or wound on the pipe with a Teflon tape, or the like, so as to be fixed onto the pipe. Accordingly, the oxide superconductors and the support member are designed so as to move as one at the time of thermal expansion or contraction. As a result, the critical current density is not reduced even in the case where the temperature changes. Thus, superconductors exhibiting excellent characteristics in repeated temperature cycles are provided. In this case, the oxide superconductors are disposed in parallel or helical with the longitudinal direction of the current lead.
On the other hand, in most cases, the high-temperature side of the current lead is constituted by a copper lead.
The conventional current lead using oxide superconductors, however, has the following problem. As the capacity of the current lead increases, that is, as the current value flowing in the current lead increases, the intensity of the self magnetic field generated by the current lead per se increases. In either a bismuth-based material or an yttrium-based material used as the superconducting material, the critical current value decreases remarkably in the magnetic field. For this reason, in a large-current-purpose current lead using a bulk-shaped superconducting material where critical current density is uniform in the material, the required sectional area of the superconducting material increases. Accordingly, there arises a disadvantage that the increase of the required sectional area causes not only increase in size of the superconducting current lead portion and complication in structure of the superconducting current lead portion, but also increase in quantity of heat entrance. In the prior art, attention has been never paid to the problem in the lowering of the critical current value caused by the self magnetic field.